Optical line of sight pointing and stabilization system

ABSTRACT

A system for stabilizing an optical line of sight. An optical system including primary optics and relay optics includes a jitter rejection mirror located within the path of the relay optics. An auto alignment system is provided for maintaining alignment of the jitter rejection mirror in response to a control signal. An auto alignment sensor detects jitter in a reference beam passing through the jitter rejection mirror, and the generated control signal is used to reduce the jitter. The reference beam is supplied by a stabilized source of laser signals which are received by the primary optics, and relayed to the jitter rejection mirror.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a system for pointing andstabilizing an optical axis of an optical system. Specifically, a systemis provided which permits correction of jitter in an optical systemusing a separate reference laser beam.

[0002] Optical imaging systems and lasing systems are available toprovide a magnified image for viewing, and/or for projecting a precisionlaser for illuminating a distant target. These systems have an opticalaxis (bore sight) which is positionable in elevation and azimuth. Theoptical system is susceptible to vibrational forces which tend to imposea jitter on any optical signal being processed by the system. Thissubstantially random motion of the optical system axis produces blurringof an image being magnified by the system. In the case of a lasingsystem, the motion imparted to the laser disturbs its pointingdirection.

[0003] These vibrational influences can be minimized by stabilizing theplatform supporting the optical system. The platform supporting thepositionable optical system, such as the tripod of a camera or therobotic arm of a surgical laser, have stabilization systems which detectvibrational displacements of the platform, and attempt to apply acounterforce to the platform to oppose the vibrational displacements.However, there are certain performance limitations in this approach. Forinstance, high performance optical systems, having many opticalelements, may have individual elements being disposed with differentlevels with respect to each other due to the vibrational forcesresulting in vibration of the optical axis of the system.

[0004] Optical systems, therefore, have an additional requirement thatnot only is the platform supporting the system stabilized, but that theoptical axis lying along the optical axis or optical line of sight bestable, so that images viewed from the system are stable, as well asmaintaining the pointing position of any laser transmission systemstable vis-à-vis the optical line of sight. The present invention isdirected to maintaining the stability of the optical line of sight in amulti-element optical system.

[0005] In order to keep the optical line of sight stable, an inertialreference must be provided which identifies any apparent motion in theline of sight and which is corrected. In space applications, a star iscommonly used as an optical inertial reference because there is noapparent motion relative to the Earth. A star, however, does not work asan inertial reference inside the Earth's atmosphere because theatmosphere moves, and, therefore, anything viewed through the atmospheretends to move.

[0006] Accordingly, in order to create the inertial reference and use itto maintain the pointing attitude stability of the optical line ofsight, the present invention has been provided.

SUMMARY OF THE INVENTION

[0007] The present invention provides a system for pointing andstabilizing an optical line of sight in an optical system. The opticalsystem includes a set of primary optics and relay optics which can beused to magnify an image received on one end thereof, or to transmit alaser to a precise location. A jitter rejection mirror is located in thepath of the optical system, preferably near the point at which an imageis viewed, or in which a laser originates in a laser pointing system.The jitter rejection mirror is positionable in response to an errorsignal generated by detecting a misalignment due to jitter between theoptical system bore axis and a reference axis. The mirror is displacedin a direction to oppose any apparent change in the optical bore sightdue to jitter.

[0008] In carrying out the invention in accordance with a preferredembodiment, changes in an inertially stable reference laser beamoriginating at the object side of the optical system is detected by anauto alignment sensor at the opposite end of the optical system. Jitterimposed on the optical system displaces the reference laser signal whichis detected by the auto alignment sensor to generate a correction signalto position the jitter rejection mirror in a direction to cancel thejitter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The above-mentioned objects and advantages of the presentinvention will be more clearly understood when considered in conjunctionwith the accompanying drawings, in which:

[0010]FIG. 1 illustrates a stabilized optical system in accordance witha preferred embodiment of the invention;

[0011]FIG. 2 illustrates the control system used in the stabilizedsystem of FIG. 1;

[0012]FIG. 3 illustrates the optical jitter correction obtainable usingthe system of FIG. 1;

[0013]FIG. 4A illustrates the top view of the stabilized inertialreference unit for generating the line of sight reference laser beam;

[0014]FIG. 4B illustrates the side view of the inertial reference unit;

[0015]FIG. 4C illustrates the plan view of the inertial reference unit;

[0016]FIG. 5A illustrates the top view of the inertial reference unithaving a mirror instead of a stabilized reference laser;

[0017]FIG. 5B illustrates the side view of the inertial reference unit;

[0018]FIG. 5C illustrates the plan view of the inertial reference unit;

[0019]FIG. 6 illustrates a separate control system utilized to stabilizethe inertial reference unit; and

[0020]FIG. 7 illustrates the control system for correcting the positionof the optical system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0021] Referring now to FIG. 1, an optical system having a stabilizedline of sight is disclosed in accordance with a preferred embodiment ofthe invention. The system includes primary optics comprising a reflector11 and sub-reflector 12. The primary optics may be used in an imagingsystem, wherein incoming imaging radiation is received on the reflector11 and sub-reflector 12, and forwarded via relay optics 13, 17, 18, 19,and 20 to an imaging sensor 24. The system shown can also be implementedas a lasing system, for accurately pointing a laser beam 22 originatingfrom laser source 25. The laser beam 22 is relayed via the relay opticsto the primary optics, and precisely pointed in accordance with theorientation of the primary optics. The primary optics are supported on agimballed system 16 so that they can be pointed within an arbitraryfield of regard.

[0022] In either application, a source of image distortion, as well as apointing error in a lasing system, results from vibrational disturbancesincident to the optical system. These disturbances may operate ondifferent parts of the optical system producing different relativedisplacements with respect to other components of the optical system,disturbing the optical line of sight (LOS) of the system. In thesesystems, it is important that the optical LOS remain stable, or thequality of the images received, or precision of pointing of the laserbeam will be compromised.

[0023] In accordance with the present invention, the system isstabilized using an inertial reference unit (IRU) 15 which generates areference laser beam 23 which is fed into the objective side of theprimary optics by extended corner cube 14. The reference laser beam 23traverses the optical system and is also subject to the same vibrationalforces as imaging radiation 10 or pointing beam 22 and experiencesjitter with respect to the optical axis of the system. The relativedisplacement of the reference beam is measured by a two-dimensionaloptical position detector 27, which is at the end of the optical pathfor the system. Displacements detected by sensor 27 are used by autoalignment controller 28 to control the position of a jitter rejectionmirror 19. Auto alignment controller 28 is a servo mechanism controllerwhich operates to control the servo controlled jitter rejection mirror19 in a direction to effectively cancel changes in the optical LOS ofthe system resulting from jitter. The sensor 27 may also be stabilizedwith its own position stabilizing system. Thus, the reference beam 23,which originates from a positionally stable source 15, produces anaccurate measurement on sensor 27 of displacement of the optical LOS ofthe system, which is used to stabilize the LOS.

[0024] The inertial reference unit 15 operates as a reference similar tothe way a star may be used in space applications because of its apparentpositional stability with respect to Earth. The inertial reference unit15 is stabilized, with its own auto controlled platform, so that anychanges in the beam position with respect to the LOS are due entirely tothe jitter induced by external forces operating on the optical systemplatform.

[0025] The attitude of the inertial pointing reference beam 23 iscalibrated and initialized in inertial space. Initial reference unit 15has its own steerable platform and can be commanded to point in anydirection in space. The inertial reference unit base is mounted on theprimary mirror 11, and the reference beam 23 is aligned to coincide withthe mirror's 11 axis. The angle between the platform of the inertialreference unit 15 and its base represents the difference between currentand desired optical LOS. The difference can be used as an error signalto control a gimbal pointing controller 16 to drive the optical systemsupporting the primary optics 11 and 12 to obtain the correct LOS.

[0026] The errors in the optical LOS system can be characterizedaccording to their temporal frequency content. The resulting errorconsists of jitter, bias, and drift. Bias and drift are characteristicsrepresenting the best straight line fit to the total position error, andare an indication of the pointing accuracy. The higher frequency errorcomponents are jitter, imposed by vibrational forces on the opticalsystem. Jitter can be defined as the standard deviation of the remainingerror once bias and drift are subtracted.

[0027] Referring now to FIG. 2, the control system for stabilizing theoptical system of FIG. 1 is shown. The optical input scene received bythe camera includes jitter from the relay optics along the optical axisof the system. The jitter rejection system 30 reduces the line of sightjitter to produce a stabilized image scene at the imaging sensor 32. Thejitter rejection system 30 includes the jitter rejection mirror and theassociated positional components 28, and sensor 27.

[0028] The inertial reference unit 15 is stabilized by inertial angularrate detectors, as will be described more particularly with respect toFIGS. 4 and 5. These rate signals are combined with attitude commands inan attitude correction network 38. The corrected attitude forcontrolling the inertial reference unit 15 position is filtered inKalman filter 37, and time optimal servo (TOS) 36 provides an inner rateloop 34 within the attitude/line of sight control loop. This results ina correctly pointed and stable inertial reference unit 15. Additionally,the position of the inertial reference unit 15 is used to generategimbal offload signals, constituting relative angular positions of theinertial reference unit to the gimbal pointing controller 16 of theoptical system of FIG. 1, to position the optical system.

[0029] By stabilizing the LOS, an improvement in the pointing of theoptical instrument is obtained, as is shown more particularly in FIG. 3.FIG. 3 illustrates the optical jitter, arc sec..2/Hz over the angularfrequency bandwidth, with both stabilization, and an unstabilizedoptical system. The lower curve represents the results of stabilizingthe optical system by jitter rejection.

[0030]FIGS. 4A, 4B, and 4C illustrate the structure used to stabilizethe inertial reference unit. The inertial reference unit includes aplurality of sensors 41, 42, 43, and 44 which measure the inertialangular rate of the platform 40. Platform 40, in turn, supports theoptical collimator of a laser system having a reference laser source 50.The optical collimator 45 directs the precision beam to the input of themain optical system, where it is used as a reference beam.

[0031] The platform is stabilized by a plurality of actuators, two ofwhich, 47 and 48, are shown. As the motion of the stabilized platformare measured via the sensors 41, 42, 43, and 44, the control electronics51 generate control signals for actuators 47, 48. The actuators 47, 48include linear displacement sensors which remeasure the relativedisplacement between platform 40 and the base of inertial reference unit15. The actuators, in turn, apply forces counter to detected vibrationalforces to the platform 40 thereby stabilizing the platform. The platformcontrol system thus formed operates in accordance with the configurationof FIG. 2. The entire structure is supported on the primary opticalreflector 11 and is steerable by attitude commands.

[0032] The sensors 41-44 for detecting the inertial angular rates of theplatform 40 require both a low frequency response as well as a highfrequency response. It is contemplated that each sensor 41-44 maycomprise two sensors, one of which is designed for measurement of lowdisplacements rates, and the other of which is designed to measure highdisplacement rates. The output of each pair of sensors can be blended toprovide a broad bandwidth detection of the inertial displacements of theplatform 40.

[0033] The foregoing design provides a two degree of freedom flexure,connecting the stabilized platform to the base of the inertial referenceunit 15. This flexure allows for rotation about the tip and tilt axes ofthe stable platform. It is rigid to all three directions oftranslations, and to rotation about the optical axis.

[0034]FIG. 5 illustrates a second embodiment of generating a referencelaser beam using a stabilized mirror. In the embodiment shown, a mirror46 replaces the collimation optics 45 of FIG. 4. The stabilized platformnow constitutes a mirror on which the reference laser can be reflectedback to the auto alignment sensor 27.

[0035]FIG. 6 is a more detailed illustration of the control systemarchitecture for the inertial reference unit 15. The system includes twocontrol loops so that the stable platform 40 remains motionless ininertial space when attitude comans are zero. It is contemplated thattwo sensors will be employed, DC sensor 61 and ARS-12 sensor 62(available from A-Tech Corporation). The two sensors 61, 62 measure thehigh and low frequency content, respectively, of displacementdisturbances incident to the platform 40. Blending filter 64 is shownwhich blends the output to provide a single error signal representingthe disturbance displacements sensed on platform 40. The blending filter64 output is combined with pointing or attitude commands in a summingjunction 54. The inertial reference unit controller 55 then positionsthe stable platform 60 to assume a position set by the attitudecommands, and be stable against the forces 56 as transmitted through theflexure to the platform 40.

[0036] An E/U core linear voltage differential transformer sensor (LVDT)63 is used to measure the position of the stabilized platform relativeto the inertial reference unit base. This position error is used todrive the gimbal pointing controller 16 for positioning the base of theoptical system to eliminate the error. Thus, the primary optics, as wellas the inertial reference unit, assumes the same LOS.

[0037] It is also considered possible to implement the inertialreference unit 15 using inertial sensors not mounted on the bottom sideof the platform. In this arrangement, the angular rate sensors aremounted to the base. The foregoing strap-down approach generatesinertial motion signals which are used to provide the disturbancecancellation on the platform surface.

[0038] An additional control system is used to control the jitter mirrorand its respective control system, as was described with respect to FIG.1, to compensate for LOS errors. The control system is shown moreparticularly in FIG. 7. Referring now to FIG. 7, the auto alignmentsensors 27 generate a two-dimensional displacement image, representingthe position of the reference laser beam. The position is used bycontroller 28 to generate signals for mirror controller 72. Thesesignals are, in turn, stabilized with feedback signals from the mirrorposition sensing sensors 74, 75, and 76. As was in the case of theinertial reference unit stabilization system, two sensors 74 and 75 areused to obtain a wide bandwidth detection of mirror displacements. Ablending filter 77 combines DC sensor and high frequency sensors 74, 75outputs to produce the error correction signal for summing junction 71.The system also includes position feedback, from the E/U core LVDT 76.Using both rate feedback, and position feed back,: combined in summingjunction 70, it is possible to obtain stable control over the mirrorposition 73. Mirror 19, as was disclosed with respect to FIG. 1, cancelsthe disturbances induced on the reference beam 23, in accordance withthe signal sensed by the auto alignment sensor 27.

[0039] Thus, there has been disclosed with respect to the one embodimentits illustration and description. Additionally, the disclosure shows anddescribes only the preferred embodiments of the invention, but is to beunderstood that the invention of capable of use in various othercombinations, modifications, and environments and is capable of changesor modifications within the scope of the inventive concept as expressedherein, commensurate with the above teachings and/or the skill orknowledge of the relevant art. The embodiments described hereinabove arefurther intended to explain best modes known of practicing the inventionand to enable others skilled in the art to utilize the invention insuch, or other, embodiments and with the various modifications requiredby the particular applications or uses of the invention. Accordingly,the description is not intended to limit the invention to the formdisclosed herein. Also, it is intended that the appended claims beconstrued to include alternative embodiments.

What is claimed is:
 1. A system for stabilizing an optical line of sightcomprising: an optical system including primary optics and relay optics;a jitter rejection mirror located in the path of said relay optics; ajitter reaction system for maintaining alignment of said jitterrejection mirror in response to a control signal; an auto alignmentsensor for detecting jitter in a laser reflecting from said jitterrejection mirror, and generating a control signal for said jitterrejection system for reducing said jitter; and a stabilized source ofreference laser which is received by said primary optics and relayed tosaid jitter rejection mirror and to said auto alignment sensor, wherebyan optical system jitter error signal is produced for controlling saidjitter rejection system.
 2. The system according to claim 1 wherein saidstabilized source of laser signals is supported on a gimbaled platformwhich is controlled to stabilize said source of laser signals.
 3. Thesystem according to claim 1 further comprising: mirror optics forseparating an image incident to said jitter rejection mirror from saidlaser signals incident to said jitter rejection mirror.
 4. The systemaccording to claim 1 further comprising: a source of laser energydirected to said optical system for producing a radiant lasing beamwhich is substantially jitter free.
 5. The system according to claim 1further comprising: a mirror for separating an image in said relayoptics from said reference laser beam; and an imaging camera positionedto receive an image from said mirror.
 6. The system according to claim 1wherein said primary optics includes a subreflector.
 7. A stabilizedsource of laser signals for providing a reference line of sight for anoptical system comprising: a platform supported by a gimbal system; aplurality of sensors for detecting the inertial motion of said platform;a plurality of actuators connected to displace said platform position;and a controller connected to said sensors and said actuators fordisplacing said platform in a direction to cancel vibration incident tosaid platform.
 8. The stabilized source of laser radiation according toclaim 7 wherein said sensors include both low frequency and highfrequency detectors having a blended output wherein a wideband signal isproduced representing said inertial motion of said platform.
 9. Thesystem for stabilizing according to claim 1 wherein said auto alignmentsensor senses the deviation of said laser radiation from an opticalboresight of said optical system.
 10. The system for stabilizingaccording to claim 1 wherein said auto alignment sensor, jitterrejection mirror and said optical system share a common optical axis.11. The system for stabilizing according to claim 2 wherein saidstabilized source is mounted to the platform containing said opticalsystem, and produces an error signal proportional to the differencebetween its optical axis and a desired optical axis of said opticalsystem.
 12. The system for stabilizing according to claim 11 whereinsaid, platform containing said optical system is supported on a servocontrolled platform and is positionable with respect to said errorsignal whereby said optical system pointing direction is controlled bythe pointing direction of said stabilized source of laser radiation.